Rotator cuff tears (RCTs) are an extremely common cause of shoulder pain and disability. Up to 20% of patients greater than the age of 50 years of age have evidence of a rotator cuff tear. In addition, patients with asymptomatic cuff tears tend to progress to larger, symptomatic tears. The outcomes of surgical repair of small RCT are good, but there has been limited success in the surgical treatment of massive RCTs. Massive RCT have been found to be associated with atrophy of the rotator cuff muscles. In addition, massive RCT are associated with the development of fatty infiltration, a phenomenon that appears to be unique to the rotator cuff in humans. Importantly, patients with large RCT with atrophy and fatty infiltration have poorer clinical outcomes than those that do not have atrophy and fatty infiltration. Thus, it appears that the natural history of large rotator cuff tears is th development of muscle atrophy and fatty infiltration, which leads to poor patient outcomes. The molecular mechanisms that lead to the development of rotator cuff muscle atrophy and fatty infiltration have not been defined. The purpose of this study is to evaluate the role of specific pathways that are critical for the development of atrophy and fatty infiltration in an animal model of rotator cuff tears. We will establish the likely pathways critical to the development of muscle atrophy and fatty infiltration. We will subsequently determine how these pathways are modified by muscle denervation, since this feature is likely a key modifier of the relevant pathways. We will focus on key pathways that have been found in other models to regulate muscle atrophy and expression of fat related genes. Specifically, we will evaluate the Akt/mTOR pathway as it relates to muscle atrophy, and the PPARy-gamma pathways are they relate to fatty infiltration. Understanding these mechanisms may allow for therapeutic modalities that would allow for inhibition or even reversal of the fatty infiltration and atrophic process following repair of massve RCTs. PUBLIC HEALTH RELEVANCE: Rotator cuff tears are one of the most common orthopaedic conditions, affecting up to 20% of patients greater than 50 years of age. Large rotator cuff tears do not heal, result in poor shoulder function, and lead to irreversible muscle changes. The purpose of this study is to understand the expression of specific proteins that lead to the irreversible muscle changes found in the setting of large rotator cuff tears.